RFR: 8269476: Skip nmethod entry barrier if there is no oops in the jit code [v2]

[王超]

> Lots of c1 and c2 jit methods do not contain any oop, so the nmethod entry barrier can be skipped.
> 
> 1, c1 jit code will patch oops or Klass into the nmethod, so the entry barrier cannot directly be eliminated, current implementation uses a jump instruction to replace the jcc instruction. If the jit code is patched to contain oops, the entry barrier is patched back to the jcc instruction.
> 
> 2, only the jit code of core library methods do not contain any oops.
> 
> 3, currently only support zgc

王超 has updated the pull request incrementally with one additional commit since the last revision:

  Only implement in x86

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/4610/files
  - new: https://git.openjdk.java.net/jdk/pull/4610/files/aca47946..51871984

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=4610&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=4610&range=00-01

  Stats: 21 lines in 6 files changed: 0 ins; 20 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4610.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4610/head:pull/4610

PR: https://git.openjdk.java.net/jdk/pull/4610